The Panel switching system was an early type of automatic telephone exchange for primarily urban service, first introduced in the Bell System in the 1920s and replaced by modern systems during the 1970s. It was developed by Western Electric Labs, later called Bell Labs, in the U.S. in parallel with the Rotary system at International Western Electric in Belgium before World War I which was used in Europe. Both systems had many features in common.
The first Panel exchange was installed at the Mulberry Central Office in Newark, New Jersey. It was placed in service on January 16, 1915. It was a semi-automatic system using telephones without a dial. Operators answered calls and keyed the station number into the panel switch. The next installation was in the Waverly central office on June 12 of the same year, also in Newark.
The Panel Machine Switching System was named for its tall panels covered with 500 rows of terminals. Each panel had an electric motor to drive its, usually sixty, selectors by electromagnetically controlled clutches. The selector was similar in effect to a stepping switch though it moved continuously rather than in steps. Each selector had five brushes, each of which could select from 100 terminals arranged in groups. Pulses were sent back from the selector to a register, which had received the dialed digits, rather than forward as in the Strowger switch step-by-step (SXS) system, hence the signaling was called revertive pulsing.
As in the SXS system, each installed office could handle up to 10,000 numbered lines, requiring four digits for a single subscriber station.
The panel system was designed to interconnect the offices of a city or a local calling area. Each office was assigned a two-digit code (later three) office prefix. Callers dialed the office code followed by the station number. In most situations this led to six-digit (later seven) numbers. But from the beginning the panel system handled seven-digit numbers (later eight), for two reasons. Party line numbers were listed with one of the letters J, M, R, and W following the line number. The caller dialed the office code, the line number, and the digit corresponding to the letter. The panel system was designed to work with manual offices of up to 10,500 lines. To call a line in a manual office, callers dialed the office code followed by the line number. For lines 10,000 and up, callers therefore dialed the office code and a five-digit line number.
As in the divided-multiple telephone switchboard arrangements with which it served until it replaced them, the panel system consisted of an originating section and a terminating section, connected by a line circuit. As in the switchboard's "A Board", the originating section was organized into "panel and jack." At first, a panel system was used only to replace the "B Board". All telephone lines were connected to the incoming section for incoming calls, while operators at the cord boards continued to handle originating calls. The line circuit consisted of a line relay to indicate that a customer had gone off-hook, and a cutoff relay to keep the line relay from interfering with an established connection. The cutoff relay was controlled by a sleeve lead that, as with the multiple switchboard, could be activated by either the incoming section or the outgoing. The incoming final selector performed a sleeve test to detect a busy line. In the original "Ground Cutoff" (GCO) version of Panel, battery on the sleeve lead indicated that busy tone was to be returned to the caller. In the later, more fireproof, and more numerous "Battery Cutoff" (BCO) offices, a grounded sleeve was the busy indicator.
Supervision or line signalling was supplied by a District Circuit, similar to the plug and light cord circuit that plugged into a line's TRS phone connector on a switchboard. It supervised the calling and called party and, when both had gone on-hook, released the ground on the sleeve lead, thus releasing all selectors, which returned down to their start position to make ready for further traffic. Some District Frames were equipped with the more complex supervisory and timing circuits required to generate coin collect and return signals and otherwise handle payphones.
Many of the urban and commercial areas where Panel was first used had message rate service rather than flat rate calling. For this reason the line finder had, besides the tip and ring leads for talking and the sleeve lead for control, a fourth wire for the District Circuit to send metering pulses to control the message register. The introduction of direct distance dialing (DDD) in the 1950s required the addition of automatic number identification equipment to allow centralized automatic message accounting.
The incoming section of the office, being fixed to the structure of the last four digits of the telephone number, had a limit of 10,000 phone numbers. In some of the urban areas where Panel was used, even a single square mile might have three or five times that many. Thus the incoming selectors of several separate switching entities shared floor space and staff, but required separate incoming trunk groups from distant offices. Sometimes an Office Select Tandem was used to distribute incoming traffic among the offices. This was a Panel office with no senders or other common control equipment; just one stage of selectors and accepting only the Office Brush and Office Group parameters. Panel Sender Tandems were also used, when their greater capabilities were worth their additional cost.
The various switching entities in the building could share the same outgoing section, and this was particularly advantageous for trunks to particularly distant parts of the city, consolidating traffic that would otherwise be scattered among smaller and less efficient trunk groups or require using a tandem switch office.
Idle outgoing trunks were picked by the traditional "sleeve test" method, as lines were, except that hunting was the usual practice for trunks rather than a special service feature. The selector moved upward through twenty terminals, checking for one with an ungrounded sleeve lead, then selecting and grounding it (this in the Battery Cutoff version, which was the later, more fireproof and more widespread one). If all the trunks were busy, the selector sent back an All Circuits Busy tone (reorder tone). There was no provision for alternate routing as in earlier manual systems and later more sophisticated mechanical ones.
While the non-director Strowger (step-by-step or SXS) switch moves synchronously with the dial pulses that come from the telephone dial, the more sophisticated Panel switch had senders, similar to the directors of later Director telephone system Strowger exchanges. The sender first registered and stored the digits that the customer dialed, and then translated the received digits into numbers appropriate for the selectors: District Brush, District Group, Office Brush, Office Group, Incoming Brush, Incoming Group, Final Brush, Final Tens, Final Units. Decoders helped by translating the first three digits of the phone number into four "District" and "Office" selecting numbers. Decoders also determined the proper rate at which to operate the message register and gave this information to the sender, which set this rate in the District Junctor. Auxiliary senders, which allowed longer telephone numbers to be dialed, were added in the mid 20th century to implement Direct Distance Dialing.
When the selector had activated the correct brush or group, the sender sent a brief open circuit signal to command the selector to stop and wait for the next number of the next stage. District and Office parameters were variable translations supplied by the decoder, while Incoming parameters and Final Brush were a fixed translation from the Thousands and Hundreds digits of the phone number, merely to adapt efficiently to the capabilities of the Panel selector.
In the 1930s when the 1XB switch crossbar switching system was introduced, it used the same Revertive Pulse register signaling system, not only to control panel selectors but to signal within itself and with similar exchanges. Later 5XB switch, 1ESS switch and other systems included RP equipment in order to maintain compatibility, in some cases decades after the last Panel switch in the city had been scrapped.
Fault detection in sender
Revertive pulsing (RP) worked faster than standard dial pulses, but the greater advantage was in problem detection. In earlier systems, when a worn pawl or other problem in a Strowger selector caused it to fail to advance, only the calling party could detect it when no connection was established. The caller eventually lost patience and redialed the call. The same user or another might get connected to the faulty selector; one bad Strowger selector could block dozens of calls per hour until subscriber complaints led staff to discover it.
With RP, the pulses were going backwards to the sender, a complex and sophisticated piece of hardware. If a selector failed to advance, it stopped sending pulses to the sender. A timer in the sender detected the failure, returned a trouble tone to the caller, held the switch train out of service with a grounded sleeve lead so no other caller could use the faulty circuit, and sounded an alarm to maintenance staff.
Calls between two panel offices, or calls within a single office, used revertive pulsing for signaling. The originating office inserted a compensating resistance during pulsing so that its loop relay sensed the same resistance for all trunks.
Revertive pulsing is a method of register signaling in which the terminating equipment sends pulses backwards to the originating equipment as it hunts for the appropriate terminal. This is in contrast to forward pulsing, in which the originating equipment directly pulses the address to the terminating side for connecting the call. In Panel offices, as the selectors were driven upwards by the motors, brushes attached to the vertical selector rods wiped over commutators at the top of the frame. These commutators contained alternating segments which served either as insulators or conductors. When the brush passed over a conductive segment, it was grounded, thereby generating a pulse which was sent back to the sender in the originating office to be counted. When the sender counted the appropriate number of pulses, it cut the power to the solenoid in the terminating office, and caused the brush to stop at the current position.
For compatibility with manual offices, two special types of signaling were supported. In areas with mostly machine switches and a few manual switchboards, Panel Call Indicator (PCI) signaling was used. PCI signaling lit lamps on the B operator's desk at the terminating manual office The lamps illuminated digits on a display panel corresponding to the number dialed. The manual operator connected the call to the appropriate jack, and then repeated the process for the next incoming call. In areas with mostly manual switches, the Call Annunciator signaling system was used to avoid installing lamp panels at every operator station. The Call Annunciator used speech recorded on strips of photographic film to verbally announce the called number to the answering operator.
PCI continued in use for tandem switching purposes, decades after its original purpose had disappeared. In the 1950s auxiliary senders were added. They allowed storing more than eight digits, and sending by Multi-frequency (MF) signaling for direct distance dialing (DDD).
Compatibility with manual offices
Panel was installed in cities where many stations still used manual (non-dial) service. Calls from panel offices to manual offices required PCI (Panel Call Indicator) signals to tell the "B Board Machine Incoming" operator or incoming operator the phone number. The called number appeared on a lit display, which allowed the operator completed the call by plugging in the appropriate jack, and ringing the subscriber.
Calls from manual offices to panel offices required the "A board" or outgoing operator to request the number from the caller, connect to an idle trunk to the distant exchange and relay to the "B Board Manual Incoming Call" operator the desired number. The "B Board Manual Incoming Call" operator then keyed in the desired number, causing the Panel machine to set up the incoming and final frames to the called telephone number.
The panel switch is an example of a power drive system, in that it used motors to drive the selectors vertically up to hunt for the desired connection, and back down again when the call was over. Strowger or crossbar systems, in contrast, use individual electromagnets for operation, and in their case the power available from an electromagnet limits the maximum size of the switch element it can move. With Panel having no such restriction, its dimensions were determined solely by the needs of the switch, and the design of the exchange, as the driving electric motor can be made as large as is necessary to move the switch elements. Thus, most calls required only about half as many stages as in earlier and later systems. Motors used on panel frames were capable of operating on alternating (AC) or direct current (DC), however they could only be started with DC. In the event of an AC power failure the motor would switch to its DC windings.
Throughout its lifetime, the Panel system was upgraded as new features became available or necessary. Such upgrades improved the initial design starting in the mid 1920s. The sender was one of the main components to receive attention. The early senders (both two- and three-digit type) stored dialed digits on rotary selector switches. The senders employed translators to convert the dialed digits into the appropriate brush and group selections needed to complete the call. As better technology became available, Panel senders were upgraded to the all-relay type. These were more reliable, and in addition, replaced the translator equipment with decoders, which also operated entirely with relays as opposed to motor-driven apparatus.
- Fagen, M. D. A History of Engineering and Science in the Bell System; The Early Years (1875–1925) Bell Telephone Laboratories (1975), p581, 607.
- ibid., p.582
- Revertive Pulsing Patent #US3875346 A, 1975
- Call switching by Panel at the Seattle Museum of Communications.
- Archive Footage of early Panel system.
- Survey of Telephone Switching
- A Panel Final Selector Frame at the Museum of Communications.
- Phone Trips: 1970s-era recordings of telephone calls involving panel switches by Evan Doorbell.
- MP3 recording of revertive pulsing, followed by machine ringing and the clicks of the Panel test line.